SCN_RING_SIZE
if (n > SCN_RING_SIZE) {
get += n - SCN_RING_SIZE;
n = SCN_RING_SIZE;
#define SCN_RING_MASK (SCN_RING_SIZE-1)
#define SCN_RING_THRESH (SCN_RING_SIZE/2)
#define SCN_RING_HIWAT (SCN_RING_SIZE - (SCN_RING_SIZE >> 2))
short sc_rbuf[SCN_RING_SIZE]; /* status + data */